Update document status input for W3C publishing to a choice value

[dschuff]

The w3c-status options correspond to the valid options for
Bikeshed's --md-status flag (i.e. Status metadata, https://speced.github.io/bikeshed/#metadata),
and refer to the W3C rec-track document
stages described in https://www.w3.org/policies/process/#maturity-stages
(Editor's Draft, Working Draft, Candidiate Recommendation Draft, and
Candidate Recommendation Snapshot).

[tlively]

Is it worth explaining the different value briefly in the description?

[rossberg]

Agreed that an explanation could be useful, if not in the description then in some appropriate doc or README in the repo.